Especially, the critical angle reads \(\sin ^2{\alpha _c}=\frac{f(r_0)}{\mu ^2_{ph}r_0^2}\) . That indicates that \(\mu \)  has a close relation with the critical deflection angle and the observer's position, which is just the phenomenon of the black hole lensing. For the light ray with the large impact parameter, its deflection angle is small. Gradually decreasing \(\mu \)  to \(\mu _{ph}\) , the deflection angle will be larger and larger, until to the bounded one \(\alpha _c\) . These behaviors are presented in Fig. REF . That is consistent with that showed in Ref. [1]}.
